Brown the Beef: Heat olive oil in a large skillet over medium-high heat. Add beef strips and cook until browned on all sides. Remove from the pan and set aside.
Make the Sauce: In the same skillet, sauté the onion until softened. Sprinkle flour over the onions and cook for 1 minute, stirring constantly. Gradually whisk in beef broth, sour cream, heavy cream, Dijon mustard, and black pepper. Bring to a boil, then reduce heat and simmer until thickened.
Combine Ingredients: Add the cooked beef back to the sauce and stir to coat.
Assemble the Casserole: In a 9x13 inch baking dish, layer the cooked egg noodles and the beef mixture.
Bake: Cover the dish with foil and bake in a preheated 350°F (175°C) oven for 20-25 minutes, or until heated through.
Serve: Garnish with fresh parsley and serve immediately.
